1. Field of the Invention
The present invention relates to Hall-effect and piezoresistive devices having striplike or platelike shape.
2. Description of the Related Art
Hall effect and certain piezoresistive devices rely upon the development ("off-diagonal") voltages perpendicular to the current due the application of a magnetic field and stress, respectively. These "off-diagonal" voltages are typically generated and measured in a four terminal (Hall plate) configuration, where a pair of contacts serves to inject the current while a second pair of contacts is used to read out the voltage developed across the device. Material inhomogeneities and mask imperfections inevitably lead to effective contact misalignments in such Hall and piezoresistive elements manufactured by photolithography, which leads to an offset voltage across the voltage contacts at null magnetic field in the Hall sensor, and at null shear stress in the piezoresistive element. This misalignment offset influences sensor performance because it is the "signal to offset" that limits sensitivity in the low magnetic field/weak stress limit, especially when the offset changes with the environment. Further complications occur when the magnetic field sensing element exhibits sensitivity to stress, and the stress sensor exhibits sensitivity to the magnetic field. Then, the magnetic field sensor tends to show an additional stress dependent voltage-offset, just as the stress sensor exhibits an additional magnetic field dependent term in the offset voltage. Under such conditions, it becomes useful to identify a technique that can both reduce the misalignment offset, and simultaneously separate the (antisymmetric off-diagonal ) Hall voltage from the (symmetric off-diagonal) piezoresistive voltage, or help cancel one of the components across the relevant contacts, such that the sensor signal reflects only the desired effects.
Silicon (Si) devices that respond to external stimuli are especially attractive because this system is conducive to the development of "smart sensors" which include on-chip signal conditioning, amplification, and logic so that the output is directly compatible with microprocessor based controllers used in electronic automation systems. The widespread availability and advanced state of Si IC technology offers, in addition, the possibility of lowcost large-scale production which, in turn, insures increased utilization of such sensors in relatively common applications. An improved magnetic sensor based on Si appears especially useful because magnetic sensors have already appeared as position detectors and contactless switches in applications ranging from electric motors to soft-drink machines and automobiles--applications where better performance would aid in reducing manufacturing tolerances elsewhere. One also feels that a high performance magnetic sensor--one which is sufficiently sensitive to detect variations in the Earth's magnetic field--would prove to be a useful component in the electronic navigation and guidance systems of future automobiles. Analogously, improved piezoresistive devices based on Silicon might find a greater role in applications such as servo-driven automobile steering systems, throttle valve position,- acceleration,- and pressure-sensing.
Magnetic sensors based on the Hall effect combine the advantage of simplicity with linearity even at low magnetic fields. Yet, the relatively low mobility in n-Si, and its piezoresistive character, have combined to reduce the impact of the Si-Hall sensor by limiting its reliability even at the 5-100 mT magnetic fields encountered in typical position sensing applications. The reduced mobility contributes to the problem by increasing the diagonal voltage drop (V.sub.XX) for a given (off-diagonal) Hall voltage (V.sub.H); misalignments in the Hall voltage contacts compound the problem by picking up a fraction of the diagonal voltage and this can become comparable to the Hall effect, producing small `signal-to-offset` at low magnetic fields. The piezoresistive character of Si contributes to the problem by allowing the generation of signals across the Hall voltage contacts, comparable in magnitude to V.sub.H, upon the application of relatively small shear stresses on the sensor. Although one might partially succeed in decoupling the sensor from external stresses in certain simple situations, the misalignment offset and the piezoresistive offset together become overwhelming in the especially hostile environments that make up most applications.
Recently, the possibility of generating a Hall effect under null (net) current has been demonstrated in GaAs-based devices, and the usefulness of this approach in compensating the misalignment offset observed in Hall sensors, independent of temperature, has been pointed out. An advantage of these double ("anti Hall bar within a Hall bar"),- and multiple,- boundary Hall effect devices, so far as offset reduction is concerned, is that it is possible to realize a vanishing current density in the vicinity of the voltage contacts, which increases insensitivity to effective voltage contact misalignments. An attempt to extend this technique to Si showed that although the misalignment offset was reduced by this technique, as expected, another contribution to the offset, an off-diagonal piezoresistive voltage in the absence of a magnetic field originating from the existence of shear stress, became substantial as compared to the Hall effect at low magnetic fields (B&lt;50 mT) for certain "high piezo" alignments within the crystal. It was noted that this off-diagonal piezoresistive voltage behaved in many respects as the Hall effect and then it became apparent that the techniques and geometries developed for misalignment offset voltage compensation in Hall effect devices could be utilized to fabricate improved sensitivity piezoresisitive devices as well. Thus, the utility of simultaneously reducing both the misalignment-offset, and separating the (antisymmetric off-diagonal) Hall voltage from the (symmetric off-diagonal) piezoresistive voltage, or cancelling insitu one of the components, also became evident.